1. Field of the Invention
The present invention relates to particle monitors used in vacuum equipment; and, in particular, the present invention relates to modular designs of particle monitors suitable for external mounting on vacuum pump lines.
2. Discussion of the Related Art
In situ particle monitors are used in many types of vacuum processing equipment, such as those used for the manufacture of VLSI integrated circuits, to detect particle levels in such equipment. One such particle monitor is disclosed in U.S. Pat. No. 5,132,548, entitled "Large Detection Area Particle Sensor for Vacuum Applications" by P. Borden et al, filed on Sep. 14, 1990, assigned to High Yield Technology Inc., and issued on Jul. 21, 1992. The principal advantage of an in situ particle monitor is that such a particle monitor provides a real-time measurement of a particle level, so that an early warning can be provided when the particle level exceed certain preset thresholds. Such early warning is valuable to minimize product loss due to particle contamination.
An in situ particle monitor is often provided in a vacuum pump line carrying gas into or out of a process chamber of a piece of vacuum processing equipment. Many integrated circuit manufacturing processes operate at intermediate pressures, i.e. those pressures which are below atmospheric but greater than a hard vacuum. Typical intermediate pressures range from 0.1 to 2 Torr. Two examples of such processes are plasma etching and chemical vapor deposition. The intermediate pressures are great enough to support particles in the gasses flowing through a process chamber of these processes. Particles are typically generated in the process chamber and are drawn into an exhaust gas line, so that a particle sensor positioned in the exhaust gas line can detect the particles carried by the effluent gas. Detecting particles using such a particle sensor is advantageous because no modification to the process chamber is required. Further, in such a configuration, the particle sensor is removed from both the bright plasma glow and the reactive chemicals which may be present in such process chamber.
A particle monitor or sensor used in an exhaust line application usually operates by laser light scattering. In such a particle monitor, a laser beam source provides a laser beam, which is focussed to travel across a pump line. Particles passing through the laser beam scatter light to photocells mounted in the vicinity of the laser beam. The photocells provide an electrical signal whenever the presence of a particle is detected. In such a particle monitor, a beam stop opposite the laser source absorbs the light in the laser beam, so as to prevent the laser beam from reflecting from the opposite wall of the pump line and impinging on the photocells.
In the prior art, an in situ particle monitor typically belongs to one of two designs. In one design, also known as a probe design, the photocells for detecting light scattering are mounted close to the laser beam and the entire sensor is inserted into the pump line. The probe design has the advantages of compactness, simplicity and high sensitivity, because of the close proximity of the photocells to the laser beam. The other design (the "external design") mounts the laser, the detection optics, and the beam stop external to the pump line, allowing the incident laser beam, as well as the scattered light, to pass through windows provided in the pump line. The external design has the advantage of removing the sensitive components of the particle monitor from the vacuum line, and is therefore more desirable when highly corrosive process effluent gasses are present, or when the gas in the line is at a temperature higher than the temperature the components of the particle sensor can tolerate.
FIG. 1 shows a particle monitor 100 of conventional external design. All components of particle monitor 100 are mounted within a box 110 that encloses pump line 104. As shown in FIG. 1, four windows 103a-103d, which are transparent to the laser light used for particle detection, are provided on pump line 104. Within box 110 is housed laser source 101, which provides a laser beam 107. Lens assembly 102 collimates and focusses laser beam 107, which enters into pump line 104 through window 103a and emerges through window 103c to terminate at laser beam stop 108. In this design, particles carried in the pump line scatter light when passing through laser beam 107. Some of the scattered light emerges from pump line 104 through window 103b. Lens 105 focusses the scattered light onto photocell 106. In FIG. 1, a black surface or mirror 109 is positioned behind window 103d on the opposite side of window 103b. Black surface or mirror 109 minimizes pick-up of stray light from the wall of pipe line 104. If not minimized, such stray light can be collected by lens 105, causing noise in photocell 108.
However, the external design suffers the disadvantage of bulk. This is because, in an external design particle sensor, the components are built around the pump line in a connected structure, so as to provide support to the particle sensor. For example, particle sensor 100, which is sensitive to vibration, is firmly held in place by box 110. However, box 110 requires considerable space, thereby limiting the choices of locations at which particle sensor 100 can be installed. Also, customizing of particle sensor 100 for various application may require box 110 be redesigned for each application. Such customization may be required to fit particle sensor 100 into certain spaces, or to accommodate difference sizes or shapes of pump line 104. Unfortunately, in many applications, little space is available at the optimal mounting location for particle detection, which is usually very near the process chamber, at or before any bend in the pump line. In many instances, the particle sensor has to be mounted further away, where it is less efficient. In some instances, such a particle monitor simply cannot be installed.